Heretofore, a molded material based on a heat-resistant polymer has been used in electrical applications requiring heat resistance. In particular, molded materials based on an aromatic polyamide (hereinafter referred to as "aramide") are advantageous industrial materials with heat resistance, chemical resistance, and flame retarding properties which are attributed to the structure of aramide molecules. Above all, tissue paper (registered trademark: NOMEX) comprising fibrid and fiber of poly(m-phenylene isophthalamide) is widely used as electrical insulating paper with excellent heat resistance.
Recently, in areas of rotary machines and transformers, there has been an increasing demand for compact machines for the purpose of reduction in the number of parts and of space-saving. As means for meeting the demand, electrical insulation materials with high thermal conductivity have been needed which can rapidly dissipate heat generated from rotary parts and coils. Conventional aramide tissue paper, which includes air in the inside, is low in thermal conductivity, and thus does not sufficiently meet the above demand.
Heretofore, as for heat dissipation sheets for heat generating electronic parts such as power transistors, thyristors, and the like, there have been proposed organo-polysiloxane (silicone resin) compositions and molded materials therefrom, based on a silicone resin with superior heat resistance mixed with a filler with good thermal conductivity. Their improvement in thermal conductivity, that could not be achieved by silicone resin alone, has been realized by way of adding a filler.
(1) Japanese Patent Application Laying-open No. 14654/1977 describes an organo-polysiloxane composition comprising organo-polysiloxane, organo-hydrogen-polysiloxane, a silica based filler, a zinc carbonate powder, and a platinum catalyst.
(2) Japanese Patent Application Laying-open No. 155506/1978 describes a heat conductive silicone rubber composition comprising organo-polysiloxane, to which are added boron nitride, a low-molecular weight organo-polysiloxane and a hardener.
(3) Japanese Patent Application Laying-open No. 2349/1981 describes a heat dissipating rubber-like molded material with improved thermal conductivity which comprises poly(di-organosiloxane) which is blended with a filler, an organic peroxide catalyst, and silicone oil, vulcanized, and molded.
As described above, silicone resin compositions and molded materials with electrical insulation properties and thermal conductivity are known in the art. However, silicone resin compositions and molded materials of (1) to (3) have been defective in that low tear strength inherent to silicone resins is not overcome and that large amounts of fillers contained therein rather deteriorate their mechanical strength.
With the aim of solving such problems, a method has been proposed in which silicone resin is reinforced with another material as follows.
(4) Japanese Patent Application Laying-open No. 21446/1983 describes a production method for a heat conductive insulation sheet in which a mixture of an addition polymerization type liquid silicone rubber and a filler for enhancing its thermal conductivity is sheet-coated on a reinforcing fabric or non-woven fabric, and then cross-linked. Here, the fabric or non-woven fabric may comprise one of inorganic fibers such as glass fiber, silica fiber, alumina fiber, or boron nitride fiber, or organic fibers such as those of polyester or fluororesin.
(5) Japanese Patent Application Laying-open No. 219034/1983 describes a production method for an electrical insulating heat dissipation rubber sheet in which a fluid silicone rubber composition containing alumina powder is coated on a reticulated insulation material and then cured. Here, the reticulated insulation material includes fabrics, knits, non-woven fabrics, and laminates thereof, and specific examples of which includes, for example, glass fiber, asbestos fiber, and silicon carbide fiber which have heat resistance matching for that of the silicone resin.
In any of (4) and (5) above, use is made of a silicone resin of good fluidity and low viscosity in order to promote integration with the reinforcing material. As described above, it is known in the art, in the production of a silicone resin sheet with a high thermal conductivity, to make an improvement in tear strength of the sheet by reinforcing it with a fabric of fiber or non-woven fabric.
However, these production methods have been expected to be useful in the application of heat dissipation of heat generating electronic parts such as power transistors, thyristors, and the like and therefore cannot always be applied to electrical insulation materials for electrical apparatuses such as rotary machines and transformers. In the case of electrical insulation materials for rotary machines and transformers, the electrical insulation characteristic as well as the following characteristics are particularly important.